The invention relates to the measurement of the frequency of a signal buried in background noise and it is suitable for use in metrology, in particular radar, velocity meters and systems including a phase-locked loop where the difference between the frequency of a useful signal and a reference frequency should be measured.
The mathematical expression of the mean number of zero-crossings of a sine signal buried in Gaussian noise is well known. However, this expression includes, in addition to the frequency, two other parameters, the pass band of the filter which is applied to the process, a priori known, and the signal-to-noise ratio, which is unknown in most cases; consequently that type of analysis is hardly useful in most cases. Moreover, there is no suitable method of evaluating the error in measuring the frequency by counting since the error too depends on the above-mentioned parameters.
Consequently, the prior art methods make it possible to measure the frequency of a signal accompanied by noise when the signal-to-noise ratio is low (less than 0 dB) only through the use of elaborate indirect techniques (fast Fourier transform, correlation) which alone give the actual value of the signal-to-noise ratio and the frequency. Reference may be had in that respect to IEEE Transactions on aerospace and electronic systems, Vol. AES-10, No. 3, May 1974, pp. 364-371; U.S. Pat. No. 3,094,666 (Smith); and Telecommunications and radio engineering, Vol. 25, No. 5, May 1970, pp. 84-88.